Diverse origins of peptidoglycan biosynthesis enzymes in Glaucophyta and Viridiplantae

Chloroplast peptidoglycan is considered a remnant inherited from the ancestral cyanobacterial endosymbionts and has served as visual evidence for the endosymbiotic theory of chloroplasts. While peptidoglycan has been identified in the glaucophyte Cyanophora paradoxa and the moss Physcomitrium patens, it is absent in red algae. To clarify the origins and phylogenetic relationship of peptidoglycan in various plant and algal groups, we examined the eleven major enzymes involved in peptidoglycan synthesis across the genomic data of 60 species within the Archaeplastida. Our findings revealed that peptidoglycan synthesis enzymes were present in many species of Glaucophyta and Viridiplantae. A complete set of eleven enzymes was found in many species of Streptophyta and Chlorophyta among green plants. Phylogenetic analysis indicated that Glaucophyta and Viridiplantae are monophyletic in the trees of MurA, MraY, and MurJ, which are derived from gene transfers from Cyanobacteria. The two lineages are closely related but not monophyletic in the PBP1 tree, which originated from Cyanobacteria/Melainabacteria. The two lineages were also monophyletic in the trees of MurD and MurE, though these enzymes did not originate from Cyanobacteria. The origins of the other enzymes were more diverse: those from Glaucophyta and Viridiplantae were not monophyletic and had various bacterial origins. These results suggest that peptidoglycan is no longer evidence for the endosymbiotic theory of chloroplast origin. We discuss potential scenarios for how peptidoglycan synthesis enzymes might have been acquired, depending on whether we assume or do not assume a cyanobacterial origin of chloroplasts.